Power density and component costs are key performance metrics of both isolated and non-isolated DC-DC power converters to provide the smallest possible physical size and/or lowest costs for a given output power requirement or specification. Resonant power converters are particularly useful for high switching frequencies such as frequencies above 1 MHz where switching losses of standard SMPS topologies (Buck, Boost etc.) tend to be unacceptable for conversion efficiency reasons. High switching frequencies are generally desirable because of the resulting decrease of the electrical and physical size of circuit components of the power converter like inductors and capacitors. The smaller components allow increase of the power density of the DC-DC power converter. In a resonant power converter an input “chopper” semiconductor switch (often MOSFET or IGBT) of the standard SMPS is replaced by a “resonant” semiconductor switch. The resonant semiconductor switch relies on resonances of a resonant network typically involving various circuit capacitances and inductances to shape the waveform of either the current or the voltage across the semiconductor switch such that, when state switching takes place, there is no current through or no voltage across the semiconductor switch. Hence power dissipation is largely eliminated in at least some of the intrinsic capacitances or inductances of the input semiconductor switch such that a dramatic increase of the switching frequency into the VHF range becomes feasible for example to values above 30 MHz. This concept is known in the art under designations like zero voltage and/or zero current switching (ZVS and/or ZCS) operation. Commonly used switched mode power converters operating under ZVS and/or ZCS are often described as class E, class F or class DE inverters or power converters.
However, it remains a significant challenge to find suitable switching devices which can operate at switching frequencies in the VHF range and handle necessary device voltages and currents to produce the required output power to the converter load. One way to attack this challenge is to use multiple resonant DC-DC power converters with lower individual output power capability and connect these in parallel and/or series to reduce the maximum output power requirement imposed on any single resonant DC-DC power converter. If a pair of these resonant DC-DC power converters with lower output power capability is controlled so they operate with a 180 degrees phase shift several new advantages of this stacked converter configuration arises. Input ripple voltage is reduced as ripple voltages from the pair of lower power resonant converters will at least partially cancel each other. This cancellation effect reduces the need for input filtering and thereby lowers the component costs of the resonant power converter and reduces EMI emission. Furthermore, voltage ripple on the converter output voltage will also at least partially be cancelled leading to the same benefits on the output side of the resonant DC-DC power converter. The 180 degrees phase shift is normally achieved by controlling the drive signal on switch control terminals of all the switches of the pair of DC-DC power converters to produce the appropriate phase shift. This control scheme leads to a requirement for digital or advanced analog control circuitry which introduce a substantial increase of complexity of the DC-DC power converter.
The IEEE paper “VHF SERIES-INPUT PARALLEL-OUTPUT INTERLEAVED SELF-OSCILLATING RESONANT SEPIC CONVERTER”, Proceedings of ECCE, USA 2013, page 2052-2056 discloses two resonant so-called SEPIC power converters that are capacitively coupled together so the drain voltage of a MOSFET switch of a first converter drives the gate of another MOSFET switch of a second converter. The two resonant SEPIC converters may operate with a phase-shift of 180° (interleaved operation).
US 2012/0300504 A1 discloses a DC-DC converter circuit comprising a plurality of in parallel coupled resonant power converters operating in interleaved mode. The respective outputs of the plurality of resonant power converters are paralleled and the inputs are coupled in series via a capacitive divider. The plurality of parallel coupled resonant power converters are operating at the substantially same switching frequency and with some phase shift between them. The latter feature provides a lower ac current through an output capacitor.
In view of these problems and challenges associated with prior art operation of multiple serially or in parallel coupled resonant DC-DC power converters, it would be advantageous to provide a low complexity and low cost control mechanism and control device that would force multiple interconnected resonant DC-DC power converters to operate with 180 degrees phase shift, or operate with 0 degree phase shift, and thereby take advantage of the above-mentioned benefits of reduced EMI emission and lower components costs.